The semiconductor integrated circuit technology is an example of where logic level converters are utilized. The impact of semiconductor integrated circuit technology is being felt at all levels of our experience. This technology is being introduced in areas such as world-wide communications systems, sophisticated computers and personal objects like wristwatches, cameras and hand-held calculators. Integrated circuit technology is working along two main lines: one is based on the MOS field effect transistor (FET), and the other is based on the bipolar transistor approach. The primary advantage of the field effect transistor in large scale integrated circuitry resides in performance and relative ease of fabrication. The main performance advantage is low power. On the other hand, the main advantage in bipolar transistors for large scale integrated circuitry is device speed; typically, the bipolar transistor can operate ten times faster than a field effect transistor.
Frequently, in the development of large scale integrated circuitry to perform a certain function, MOS and TTL integrated circuit chips are inter-related. Accordingly, it is necessary to send digital signals which are generated in a TTL circuit to an MOS circuit, such as a p channel MOS(PMOS) circuit. This usually requires some kind of conversion process since the logic levels are not directly compatible. Typically, TTL circuits utilize positive logic. On the other hand, PMOS circuits utilize negative logic in which the voltage excursion between levels is greater than that for the TTL logic circuits.
Circuits for converting from TTL to high threshold PMOS or low threshold PMOS are known. High threshold is intended to include FETs having threshold voltages ranging from about 2.5 to 3.5 volts while low threshold is intended to include FETs having threshold voltages ranging from about 0.8 to 1.5 volts. Presently known converters designed for operation with a high threshold PMOS process drain high quiescent current, are slow, and occupy a large amount of space. A typical converter includes a field effect transistor in which the TTL logic levels are applied to the gate, and the voltage at the source is derived from a resistive voltage divider network comprising diffusion resistors. Thus, the divider is always drawing current and its use of diffusion resistors requires a large space on the chip. Further, this converter was found to not operate at adequate speeds. On the other hand, converters designed for operation with a low threshold PMOS process may have undesirable noise immunity problems.